This invention relates to a method of mounting a semiconductor element. More particularly, this invention relates to a mounting method which permits a semiconductor element to be used at higher power levels than previous power module designs permitted, by facilitating the increased cooling of said element, while at the same time providing electrical isolation of said element.
It is well known that the proper utilization of a semiconductor element requires an awareness that heat developed by power dissipation at the junction of the semiconductor must be carefully considered. This heat generation problem becomes particularly important in semiconductor applications involving one ampere of current or more--so-called "power semiconductor" applications. Short-term or long-term temperature excursions not only impair the electrical characteristics of a semiconductor device, but also set up internal mechanical stresses at each of the semiconductor material interfaces. In order to obtain maximum efficiency and greater reliability in the use of a semiconductor, heat developed by power dissipation at the junction must be removed as rapidly as possible. Power semiconductors are normally mounted in a package for purposes of protection of the delicate semiconductor structure; this package is then itself often mounted on a heat sink to facilitate cooling of the semiconductor elements. Thus, heat generated in the semiconductor elements travels from the element to the package, then to the heat sink, and then to the ambient (e.g., natural convection air, forced air, oil, water, etc.). By increasing the thermal conductance at any point along this heat transfer path, one can improve the heat transfer from, and thus the cooling of, the semiconductor element. See Westinghouse Power Semiconductor User's Manual and Data Book (January 1978), p. 42.
Power semiconductors were first offered only in stud-mount assemblies, which provided only one-side cooling of the semiconductor element. Moreover, these assemblies were limited to relatively small devices due to problems occurring in the solder used to bond the semiconductor element to the device package.
Subsequently, packaging of the usually wafer-shaped semiconductor element in a disc construction avoided the solder problem, and provided increased thermal conductance by permitting cooling of both sides of the element. Good thermal contact between the element and the encompassing disc is achieved by applying and maintaining force approximately in the range of 500 to 10,000 pounds on the parallel sides of the disc, which are thus pressed into intimate contact with the parallel sides of the semiconductor wafer. In addition, these discs devices can be readily arranged in series or back-to-back, or may be inverted to reverse polarity. However, prior art disc packages require special clamps and/or springs to produce and maintain the required pressure on the sides of the disc, resulting in a complete assembly that is relatively bulky, complicated, and expensive. Although these disc packages permit double-side cooling of the semiconductor element, they present the further serious drawback that the power connections to the semiconductor device are not electrically isolated from the heat sinks.
Therefore, it is an object of the present invention to provide a method for mounting a semiconductor element that will increase the power-handling capability of said element by providing double-side cooling thereof.
It is another object of this invention to provide such a method that will, at the same time, eliminate the need for bulky and complicated springs or clamps to maintain pressure on the semiconductor/package interface.
It is still another object of this invention to provide such a method that will also premit mounting of semiconductor element in electrical isolation from a heat sink to which it may be attached.
Still another object of this invention is to provide such a method that will enable maintenance of the requisite pressure on the semiconductor element over its entire operating temperature range.
It is a further object of this invention to provide such a method that will enable use of a single heat sink, while providing isolated, double-side cooling of the semiconductor element.
Objects and advantages of the present invention are set forth in part above and in part below. In addition, these and other objects and advantages of the invention will become apparent herefrom or may be appreciated by practice with the invention, the same being realized and attained by the instrumentalities, combinations, and methodologies pointed out in the appended claims. Accordingly, the invention resides in the novel parts, arrangements, combinations, steps, methods, and improvements herein shown and described.